1. Field of the Invention
The present invention relates to an oxide superconducting wire and a method of preparing the same, and a cable conductor which is formed by assembling such oxide superconducting wires, and more particularly, it relates to an oxide superconducting wire which can carry a heavy current in ac application and a method of preparing the same, and a cable conductor which is formed by assembling such oxide superconducting wires.
2. Description of the Background Art
The principal feature of an oxide superconductor resides in that the same is in a superconducting state also at a temperature exceeding the liquid nitrogen temperature. Therefore, a wire consisting of such an oxide superconductor is expected for application to a superconducting device, as a material which can be used under cooling with liquid nitrogen.
The inventors have developed a tape-shaped Bi-based Ag-coated multifilamentary wire, which is prepared from filaments of an oxide superconductor with a stabilizer of silver. A Bi-based Ag-coated wire can be prepared by charging a metal pipe with raw material powder serving as a precursor for a Bi oxide superconductor, wire-drawing the pipe and thereafter repeating rolling and a heat treatment a plurality of times.
On the other hand, a multifilamentary wire can be prepared by charging metal pipes with raw material powder, wire-drawing the same, engaging a plurality of such wires in a metal pipe for forming a multi-filamentary substance, further wire-drawing the same and thereafter repeating rolling and a heat treatment a plurality of times.
Among such preparation steps, the rolling step is effective for improving the orientation of crystal grains in the Bi superconductor having a plate-type crystal structure, strengthening bonding between the crystal grains and improving the density of the filaments, and regarded as being indispensable for attaining a high critical current density in preparation of a Bi-based Ag-coated wire.
Further, the aspect ratio of a section of the wire is increased by this rolling, whereby the aspect ratio of a section of each filament is also increased. This is advantageous for growth of the plate-type crystals, and a high critical current density is consequently attained.
On the other hand, the heat treatment step for the purpose of sintering is also indispensable for forming the superconductor, attaining crystal growth and strengthening bonding between the crystal grains, since the oxide superconductor is ceramics.
The Bi-based Ag-coated wire which is prepared in the aforementioned manner is excellent in bending property and capable of preparing a long wire having a critical current density exceeding 104 A/cm2, and hence the same is expected for application to a superconducting cable or magnet.
In ac application of such an oxide superconducting wire, however, ac loss resulting from a fluctuating magnetic field in driving comes into question. In a cable conductor which is formed by assembling superconducting wires, on the other hand, there arises a new problem to be solved such as a drift phenomenon resulting from ununiformity between impedances of the wires, which cannot be caused in dc application. Due to a drift caused in such a manner, further, loss upon formation of the conductor is disadvantageously increased beyond the sum of ac loss values of strands.
As to such problems caused in ac application, various countermeasures have generally been studied in relation to metal superconducting wires, for example. In more concrete terms, countermeasures of arranging high resistance barrier layers around or between filaments, preparing an extra-fine multifilamentary wire from superconducting filaments, increasing the specific resistance of a matrix and the like are studied in order to reduce ac loss. In order to suppress a current drift by uniformalizing the impedances of the filaments or wires in a conductor for an ac magnet, on the other hand, countermeasures of twisting the filaments or wires, dislocating the wires or filaments and the like are studied.
In order to attain a heavy current, further, a countermeasure of further twisting primary stranded wires each prepared by twisting superconducting strands to attain a flat-molded multinary structure or the like is studied.
While a countermeasure of further twisting primarily stranded wires to attain a multinary structure or the like must be taken also in employment of the aforementioned Bi-based Ag-coated wire for ac application similarly to the metal superconducting wire, however, it is impossible to implement the aforementioned multinary structure through an oxide superconducting wire by a method which is absolutely identical to that for the metal superconducting wire. This is because a Bi-based Ag-coated multifilamentary wire indispensably requires rolling and sintering processes as described above, while no such rolling and sintering steps are required for preparing a metal superconducting wire.
Namely, it is difficult to twist wires of a Bi oxide superconductor after sintering, since the Bi oxide superconductor is ceramics which is weak against bending distortion. Even if such wires can be twisted, a high critical current density cannot be attained. Further, it is difficult to twist wires in which aspect ratios of sections are increased by rolling. Even if such wires can be twisted, a number of clearances are defined in the stranded wire as compared with that prepared by twisting round wires, and a high critical current density cannot be attained.
In order to solve the aforementioned problems, an object of the present invention is to provide an oxide superconducting wire which maintains a high critical current density and has a small current drift with small ac loss when the same carries an alternating current and a method of preparing the same, and a cable conductor which is formed by assembling such oxide superconducting wires.
According to an aspect of the present invention, an oxide superconducting wire is provided. This oxide superconducting wire is a flat-molded stranded wire which is formed by twisting a plurality of metal-coated strands consisting of an oxide superconductor, and is characterized in that the flat-molded stranded wire has a rectangular sectional shape, and a section of each strand forming the flat-molded stranded wire has an aspect ratio of at least 2.
Throughout the specification, the term xe2x80x9caspect ratioxe2x80x9d indicates the ratio of the thickness to the width in a cross section of the oxide superconducting wire.
Superconducting filaments provided in the strands can be brought into flat shapes having a high aspect ratio by setting the strands at an aspect ratio of at least 2. Consequently, a superconducting wire having a high critical current density can be obtained. In particular, the aspect ratio of the superconducting filaments is preferably around 10. The section of each strand preferably has an aspect ratio of not more than 20. It is difficult to increase the aspect ratio of the strands beyond 20 in case of twisting and molding the same.
According to the present invention, the strands are completely dislocated due to the twisting, whereby the impedances of the strands forming the stranded wire can be equalized to each other.
According to the present invention, further, the stranded wire has a rectangular sectional shape. Thus, the wire can be densely wound to be advantageously compacted when the same is applied to a coil or a cable.
Preferably, the metal coatings of the strands consist of silver or a silver alloy, and coating layers consisting of a material having higher resistance than silver are provided on the outer peripheries of the metal coatings.
Due to the presence of such coating layers, the strands can be prevented from bonding in the stranded wire, so that ac loss is effectively reduced.
The material having higher resistance than silver is prepared from a high resistance metal material or an inorganic insulating material, for example.
When no such coating layers consisting of a material having higher resistance than silver such as a high resistance metal material or an inorganic insulating material are present, metal matrices of silver or the like are so diffused during the heat treatment that the strands are disadvantageously bonded with each other, and hence bonding loss between the strands may be increased. The coating layers having higher resistance than silver effectively function to reduce such bonding loss.
The high resistance material is prepared from an Agxe2x80x94Mn alloy, an Agxe2x80x94Au alloy, or Ni or Cr having high resistance, for example.
On the other hand, the inorganic insulating material is prepared from an oxide insulating material such as MgO or CuO which is obtained by oxidizing Mg or Cu, for example. Bonding between the strands can be completely prevented by the coating layers consisting of such an insulating material. Further, the effect of dislocation is rendered further complete.
According to another aspect of the present invention, a method of preparing an oxide superconducting wire is provided. This method comprises the steps of preparing a stranded wire by twisting a plurality of strands each formed by metal-coating an oxide superconductor or raw material powder therefor, flat-molding the prepared stranded wire, and repeating rolling and a heat treatment of at least 800xc2x0 C. on the flat-molded stranded wire a plurality of times.
Namely, a plurality of round wire type strands each formed by metal-coating an oxide superconductor or raw material powder therefor, which are not sintered as wires, are prepared. Then, the plurality of strands are twisted for preparing a stranded wire. As to the number of twisted strands, three, seven or twelve strands can be twisted, for example.
This stranded wire is flat-molded and thereafter further rolled, whereby superconducting filaments having circular sections which are provided in the strands can be deformed in the form of flat plates having a high aspect ratio. The dimensions of the superconducting filaments are preferably within the ranges of 0.1 to 100 xcexcm in thickness and 1 xcexcm to 1 mm in width. In the flat-molding, the superconducting filaments can be simultaneously deformed by application of rolling loads from above and under the wire.
Thereafter the step of performing rolling and a heat treatment is carried out at least twice, whereby an oxide superconducting wire in which strands are completely dislocated in order to cope with application to an ac wire can be obtained.
According to the present invention, the respective filaments are subjected to twisting as well as rolling. In the inventive wire, therefore, the impedances of the respective filaments are uniformalized by twisting. Also when the wire carries an alternating current, therefore, the current can be uniformly fed to the respective filaments. Further, bonding currents between the filaments are suppressed for effectively reducing ac loss. When the surfaces of the strands are insulated, it is possible to further suppress the bonding currents and reduce the ac loss.
According to the present invention, the flat-molded stranded wire rolled and heat treated. Thus, a high critical current density can also be attained by strengthening grain bonding which is broken by distortion in formation of the stranded wire and regularizing disturbed orientation.
According to the present invention, further, it is also possible to prepare a flat-molded multinary stranded wire by further twisting a plurality of primary stranded wires each obtained by twisting a plurality of strands. As to the number of twisted stranded wires, nine primary stranded wires can be twisted, for example.
It is particularly important to carry out the twisting step a plurality of times, in order to attain the aforementioned effects when the number of strands which are twisted for the purpose of attaining a high capacitance is increased.
According to the present invention, further, a stranded wire may be prepared by stacking and integrating a plurality of tape-shaped strands with each other and thereafter twisting the same. Particularly in case of a silver sheath Bi 2223 superconducting wire, it is important to prepare tape-shaped strands for attaining a high critical current density. Twisting is simplified by stacking the tape-shaped strands with each other for reducing the aspect ratio of sections thereof and thereafter twisting the same, and characteristic deterioration caused by bending distortion or the like can be effectively prevented.
The strands can be integrated with each other by a method of heat treating the stacked strands and bonding the same with each other by diffusion of silver, a method of performing compression molding, or a method of stacking the strands in a flat pipe, for example. In case of a long wire, it is effective to wire-draw and twist the strands after integrating the same with each other.
The tape-shaped strands are preferably previously heat treated in advance of twisting. It is possible to reinforce grain bonding of the oxide superconductor for attaining a high critical current density by further performing a heat treatment after forming the oxide superconductor by this heat treatment and performing the step of twisting etc.
According to the present invention, the method preferably further comprises a step of previously coating the outer peripheries of the strands with a material having higher resistance than silver before twisting the metal-coated strands for preparing the stranded wire.
The coating layers consisting of a material having higher resistance than silver can be formed by a method of adding Ni or Cr of high resistance to the outer surfaces of the strands by plating, or a method of applying a solution in which powder of an oxide insulating material such as AlO3 is dispersed to the outer surfaces of the strands, for example.
Alternatively, coating layers consisting of a metal such as Mg or Cu may be formed on the outer peripheries of the strands so that these layers are thereafter oxidized to form coating layers consisting of an oxide insulating material such as MgO or CuO. In particular, excellent workability can be attained by performing an oxidizing step after the rolling of the stranded wire. Mg or Cu is richer in workability than MgO or CuO, and hence the stranded wire can be molded and rolled into a better shape by oxidizing the coating layers after performing twisting and rolling.
According to the present invention, the method preferably further comprises a step of previously coating the flat-molded stranded wire with a metal before rolling.
If the outermost layer of a flat-molded multinary stranded wire is so thin that superconducting filaments may be exposed through the subsequent rolling step, the stranded wire is preferably coated with a metal in advance.
Metal coating layers may be further formed on the outer peripheries of the strands which are coated with a metal such as silver or a silver alloy by performing metal coating on the surface of the flat-molded multinary stranded wire or engagement in a flat metal pipe.
According to the present invention, further, each strand is preferably a multifilamentary wire which is formed by embedding a plurality of superconductors in a metal matrix. Due to a plurality of superconducting filaments provided in each strand, flexibility of the wire is improved.
According to the present invention, the strands themselves are preferably subjected to twisting. Due to such twisting of the strands themselves, bonding loss and eddy current loss are reduced thereby reducing ac loss as a result.
According to the present invention, the method preferably further comprises a step of temporarily heat treating the flat-molded stranded wire before rolling. Workability in rolling can be improved by heat treating the flat-molded stranded wire at about 800xc2x0 C. for diffusion-bonding the strands with each other.
According to the present invention, further, a step of winding strands around a core of a flat-molded stranded wire and flat-molding the same is preferably repeated a plurality of times.
A wire having low loss and a high capacitance can be obtained by repeating flat-twisting/molding a plurality of times. Such a wire is effective as a material for forming a compact cable conductor having low loss and a high capacitance.
According to still another aspect of the present invention, an oxide superconducting cable conductor is provided. This oxide superconducting cable conductor is formed by assembling oxide superconducting wires on a cylindrical former. Each oxide superconducting wire is a flat-molded stranded wire which is formed by twisting a plurality of metal-coated strands consisting of an oxide superconductor. This flat-molded stranded wire has a rectangular sectional shape, while a section of each strand forming the flat-molded stranded wire has an aspect ratio of at least 2.
In a single-layer cable conductor formed by assembling oxide superconducting wires on a former in a single layer, for example, all strands are dislocated to occupy positions which are electromagnetically completely equivalent to each other, whereby current distribution in the conductor is so uniformalized that increase of ac loss caused by a drift can be prevented. When wires are spirally wound on a former, on the other hand, it is effective to form the conductor in a two-layer structure so that first and second layers are wound in opposite directions, in order to cancel a magnetic field component along the longitudinal direction of the conductor. Thus, a drift between the layers caused by impedance difference therebetween as well as following ac loss can be minimized as compared with a multilayer conductor, by forming the conductor in a single- or two-layer structure.
According to the present invention, as hereinabove described, a metal-coated oxide superconducting wire having a high critical current density which can transmit a current with los loss can be obtained.
According to the present invention, further, it is also possible to increase the critical current per wire beyond 100 A by increasing the number of stranded wires and the degree of twisting, i.e., the number of times of twisting, so that the inventive wire is usefully applied to an oxide superconducting cable or a superconducting magnet which is employed for carrying a high capacitance alternating current.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.